1. Field of the Invention
This invention relates to a semiconductor device having a circuit comprising thin film transistors (hereinafter referred to as “TFTs”) on a substrate having an insulation surface, and to a fabrication method of such a semiconductor device. More specifically, the present invention relates to electro-optical apparatuses (called also “electronic appliances”) typified by a liquid crystal display device including a pixel unit (pixel matrix circuit) and driving circuits (driver circuits) disposed around the pixel unit and formed on the same substrate and an EL (Electro-Luminescence) display device, and electrical appliances (called also “electronic appliances”) having the electro-optical apparatus mounted thereto.
The term “semiconductor device” used in this specification represents generally those apparatuses which function by utilizing semiconductor characteristics, and includes also the electro-optical apparatuses and electrical appliances using the electro-optical apparatus described above.
2. Description of the Related Art
Development of a semiconductor device having a large area integrated circuit, that comprises TFTs formed on a substrate having an insulation surface, has been made progressively. An active matrix type liquid crystal display device, an EL display device and a close adhesion type image sensor are typical of such semiconductor devices. Particularly because TFTs using a polycrystalline silicon film (typically, a poly-Si film) as an active layer (the TFT will be hereinafter referred to as “poly-silicon TFT”) have high field mobility, they can form a variety of functional circuits.
In the active matrix type liquid crystal display device, for example, an integrated circuit that includes a pixel unit for displaying images for each functional block, a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, and so forth, each being based on a CMOS circuit, is formed on one substrate. In the case of the close adhesion type image sensor, an integrated circuit such as a sample-and-hold circuit, a shift register circuit, a multiplexer circuit, and so forth, is formed by using the TFTs.
These driving circuits (which are also called “peripheral driving circuits”) do not always have the same operating condition. Therefore, the characteristics required for the TFTs are naturally different to certain extents. The pixel unit comprises a pixel TFT functioning as a switching device and an auxiliary holding capacitance, and a voltage is applied to a liquid crystal to drive it. Here, an alternating current must be applied to drive the liquid crystal, and a system called “frame inversion driving” has gained a wide application. Therefore, one of the required characteristics of the TFT is that an OFF current value (a drain current value flowing through the TFT when it is in the OFF operation) must be sufficiently lowered. Because a high driving voltage is applied to the buffer circuit, the TFT must have a high withstand voltage such that it does not undergo breakdown even when a high voltage is applied. In order to improve the current driving capacity, it is necessary to sufficiently secure the ON current value (the drain current value flowing through the TFT when it is in the ON operation).
However, the poly-silicon TFT involves the problem that its OFF current is likely to become high. Degradation such as the drop of the ON current value is observed in the poly-silicon TFT in the same way as in MOS transistors used for ICs, or the like. It is believed that the main cause is hot carrier injection, and the hot carriers generated by a high field in the proximity of the drain presumably invite this degradation.
An LDD (Lightly Doped Drain) structure is known as a structure of the TFT for lowering the OFF current value. This structure forms an impurity region having a low concentration between a channel formation region and a source or drain region to which an impurity is doped in a high concentration. The low concentration impurity region is called the “LDD region”.
A so-called “GOLD (Gate-drain Overlapped LDD) structure” is also known as a structure for preventing deterioration of the ON current value by hot carrier injection. Since the LDD region is so arranged as to overlap with a gate wiring through a gate insulation film in this structure, this structure is effective for preventing hot carrier injection in the proximity of the drain and for improving reliability. For example, Mutsuko Hatano, Hajime Akimoto and Takeshi Sakai, “IEDM97 Technical Digest”, pp. 523-526, 1997, discloses a GOLD structure using side walls formed from silicon. It has been confirmed that this structure provides by far higher reliability than the TFTs having other structures.
In an active matrix type liquid crystal display device, a TFT is disposed for each of dozens to millions of pixels and a pixel electrode is disposed for each TFT. An opposing electrode is provided on an opposing substrate side sandwiching a liquid crystal, and forms a kind of capacitors using the liquid crystal as a dielectric. The voltage to be applied to each pixel is controlled by the switching function of the TFT. As the charge to this capacitor is controlled, the liquid crystal is driven, and an image is displayed by controlling the quantity of transmitting rays of light.
However, the accumulated capacity of this capacitor decreases gradually due to a leakage current resulting from the OFF current, or the like. Consequently, the quantity of transmitting rays of light changes, thereby lowering the contrast of image display. Therefore, it has been customary to dispose a capacitance wiring, and to arrange another capacitor (called a “holding capacitance”) in parallel with the capacitor using the liquid crystal as the dielectric in order to supplement the capacitance lost by the capacitor using the liquid crystal as the dielectric.
Nonetheless, the required characteristics of the pixel TFT of the pixel unit are not always the same as the required characteristics of the TFT (herein after called the “driving TFT”) of a logic circuit (called also the “driving circuit”) such as the shift register circuit and the buffer circuit. For example, a large reverse bias voltage (a negative voltage in n-channel TFT) is applied to the gate wiring in the pixel TFT, but the TFT of the driving circuit is not fundamentally driven by the application of the reverse bias voltage. The operation speed of the former may be lower than 1/100 of the latter.
The GOLD structure has a high effect for preventing the degradation of the ON current value, it is true, but is not free from the problem that the OFF current value becomes greater than the ordinary LDD structures. Therefore, the GOLD structure cannot be said as an entirely preferable structure for the pixel TFT, in particular. On the contrary, the ordinary LDD structures have a high effect for restricting the OFF current value, but is not resistant to hot carrier injection, as is well known in the art.
For these reasons, it is not always preferred to constitute all the TFTs by the same construction in the semiconductor devices having a plurality of integrated circuits such as the active matrix type liquid crystal display device.
When a sufficient capacitance is secured by forming a holding capacitance using the capacitance wiring in the pixel unit as represented by the prior art example described above, an aperture ratio (a ratio of an area capable of image display to an area of one pixel) must be sacrificed. Particularly in the case of a small high precision panel used for a projector type display device, the area per pixel is so small that the drop of the aperture ratio by the capacitance wiring becomes a serious problem.